The present invention relates to an absorption heat pump system.
Generally, a refrigeration cycle has a heat absorption side and a heat radiation side. When the heat absorption side is utilized, the system serves as a refrigerator, whereas, when the heat radiation side is utilized, the system serves as a heat pump. All heat pumps follow this concept regardless of whether they are of compression type or absorption type.
The absorption heat pump system of the invention, however, is an absorption type system workable only as a heat pump and, hence, is not the same as that of the above-mentioned concept. More specifically, the invention is concerned with an absorption heat pump system in which a refrigerant is evaporated by heat utilizable at a low temperature level and a warmed water of a comparatively high temperature is produced by the heat of absorption which is generated when the vapor of the refrigerant is absorbed by an absorbent, i.e., the type in which the evaporation temperature and the vapor pressure of the refrigerant in the evaporator are higher than the condensation temperature and the vapor pressure of the refrigerant in the condenser and the absorption temperature of the refrigerant is higher than the temperature at which the refrigerant is generated.
FIG. 1 shows a basic arrangement of the absorption heat pump of the kind described. As will be seen from this Figure, the absorption heat pump has an evaporator 1 and an absorber 2 accommodated by an upper section constituting the high-pressure side and a generator 3 and a condenser 4 accommodated by a lower section which constitutes the low-pressure side. These constituents are connected hermetically through a refrigerant line 6 having a first refrigerant pump 5, a refrigerant line 8 having a second refrigerant pump 7, a concentrated solution line 10 having a solution pump 9, a U-shaped dilute solution line 11 and a solution heat exchanger 12 so as to constitute an absorption heat pump cycle. Heating medium tubes 13 and 14 are provided in the evaporator 1 and the generator 3, respectively, while the condenser 4 and the absorber 2 are provided with cooling water tubes 15 and heated water tubes 16, respectively.
In the absorber 2, heat of a temperature level higher than the heating medium ciculated through the heat medium tubes 13 is produced by the energy possesed by the refrigerant gas evaporated by the heat derived from the heat medium tubes 13 and also by the reaction heat which is generated when the refrigerant gas is absorbed by the solution. According to this arrangement, therefore, it is possible to obtain warmed water or vapor of water of a temperature level higher than the low-temperature heat source is obtained in a heat exchanger 16' of the absorber 2, and to supply the warm water or the water vapor to the load 19. This system will be referred to as "hot fluid production type absorption heat pump system".
For instance, by the use of lithium bromide as the absorbent and water as the refrigerant, while using waste steam of 98.degree. C. as the low-temperature heat source and circulating cooling water of about 25.degree. C. through the condenser, it is possible to obtain water vapor of about 130.degree. C. from the heat exchanger 16' of the absorber 2.
In the described operation of the absorption heat pump system, the steady supply of the warm water or water vapor of constant temperature is achievable only under an ideal condition of operation. Namely, any fluctuation in the rate of supply of the evaporated refrigerant flowing from the evaporator 1 into the absorber 2 or in the refrigerant temperature tends to appear as a large fluctuation in the amount of heat energy supplied to the load 19. For instance, assuming that a heat pump system exhibits a thermal output fluctuation as shown in FIG. 2, this heat pump system is considered as having a capacity smaller than the average value M of the thermal output fluctuation, rather than the average value M, from the view point of heat capacity demanded by the load 19. This means that the efficiency of operation of the heat pump system is extremly low.
More practically, referring to FIG. 2 in which the vertical axis represents the output temperature and the horizontal axis represents the time, all of the thermal output at temperature levels below that is demanded by the load 19 cannot be used practically, unless the output temperature is raised by a suitable auxiliary heater which is not shown. In such a case, the heat pump system is materially unable to supply the heat to the load over most part of the operation period, i.e. the apparent output is drastically lowered, because of the fluctuation in the heating capacity even though the system inherently has sufficiently large heating capacity.
In connection with the described basic arrangement of the absorption heat pump system, it has been obliged to make an on-off control of the operation of the second refrigerant pump 7 in accordance with a signal derived from a level detector disposed in a refrigerant reservoir disposed at the lower side of the condenser 4. Moreover, in the absorption heat pump system, there is a tendency that a cavitation of the second refrigerant pump 7 takes place because of lowering of the liquid level in the liquid refrigerant reservoir attributable to a reduction in the amount of the condensed refrigerant in the condenser 4 as a result of a reduction in the rate of generation of the refrigerant vapor in the generator 3 which in turn takes place when the rate of energy supplied by the heat source is decreased, i.e. when there is a reduction in the rate of supply or the temperature of the heat source fluid such as waste warm water or steam from factories or power plants, warm water heated by solar energy and so forth. In order to avoid the cavitation, such an on-off control of the second refrigerant pump 7 is conducted in accordance with the signal from a level detector for detecting the level of liquid refrigerant in the liquid refrigerant reservoir in such a manner that, when the liquid level has come down below a predetermined level, the second refrigerant pump 7 is stopped but the same is started again as the level of the liquid refrigerant has been increased beyond the predetermined level.
This conventionally adopted on-off control, however, imposes various problems. For instance, when the second refrigerant pump is started again, a large amount of condensate liquid refrigerant of low temperature is introduced from the condenser 4 to the evaporator 1 of high-pressure and temperature side, so that the pressure and temperature in the upper section are drastically lowered to inconveniently lower the temperature of the warm water supplied by the heat pump system. Namely, a so-called hunting of the output temperature inevitably takes place in response to the repeated starting and stopping of the second refrigerant pump 7.
In the conventional absorption heat pump system of the type described, the pressure differential between the upper section of high pressure and the lower section of low pressure is decreased as the temperature of the cooling water circulated through the condenser 4 is raised or as the heat input to the evaporator 1 is reduced, whereas the discharge pressure of the second refrigerant pump is not changed substantially. In consequence, the rate of supply of the liquid refrigerant from the condenser 4 to the evaporator 1 is increased to cause a drastic lowering of the liquid level in the liquid refrigerant reservoir 18. Since the on-off control of the second refrigerant pump 7 is conducted in response to this drastic change in the liquid level, the output temperature, i.e. the temperature of warm water produced in the system, is unstabilized. In addition, the efficiency of operation of the absorption heat pump system is decreased due to escape of the refrigerant liquid into a solution reservoir 20 below the absorber 2, partly because a rise in the liquid level in the unevaporated refrigerant reservoir 19 under the evaporator due to a decrease in the rate of evaporation of refrigerant caused by a reduction in the heat input to the evaporator 1, and partly because a large amount of condensate refrigerant liquid is introduced at once from the condenser 4 into the evaporator 1. As a result, the capacity of the system for supplying the warm water, i.e. the warm water output of the system, is further decreased.
Another problem in the hot fluid production type absorption heat pump system is that, when the temperature or flow rate of the liquid refrigerant introduced from the condenser to the evaporator is decreased, the rate of evaporation of the refrigerant is decreased as a result of lowering of the refrigerant temperature in the evaporator, because of the fact that the condensation temperature of the refrigerant in the condenser is lower than the evaporation temperature of the same in the evaporator.
As a countermeasure for obviating this problem, it is of course advisable to control directly the major factors, i.e. the temperature and flow rate of the heat source medium supplied to the evaporator and the generator.
In the heat pump system of the hot fluid production type, however, the waste heat fluid of a comparatively low temperature level, discharged from production equipments such as factories, chemical processes or the like is used as the heat source fluid supplied to the evaporator and generator. Under this circumstance, there is a practical limit in controlling the temperature and flow rate of the heat source fluid supplied to the evaporator and the generator, from the view point of the operation efficiency of the production equipment from which the heat source fluid is derived. The same problem, i.e. the difficulty in effecting direct control of the flow rate and temperature of the heat source fluid, is encountered also when a geo-thermal heating medium such as hot water or steam in springs is utilized as the heat source medium.